Plasma cutting method and NC plasma cutting apparatus

ABSTRACT

The present invention relates to a plasma cutting method and an NC plasma cutting apparatus in which an improvement can be made in the quality of a bore-cut product obtained by using a transferred plasma arc, and the productive efficiency can also be enhanced. In order to achieve the above-described advantages, the transferred plasma arc is switched to a non-transferred plasma arc at a position immediately before a cutting finishing point and in the vicinity of a position at which cutting lines cross each other. Simultaneously with this switching operation, the moving speed of the plasma torch is varied in accordance with a change in the plasma arc current. Additionally, the transferred plasma arc current is reduced at a position immediately before the cutting finishing point and in the vicinity of a position at which cutting lines cross each other. The moving speed of the plasma torch can be varied in accordance with the reduction in the arc current. Further, the above-described voltage detecting function is kept in the off state during a cutting operation, and it is actuated at a position immediately before the cutting finishing point and in the vicinity of a position at which cutting lines cross each other.

TECHNICAL FIELD

The present invention relates to a plasma cutting method and an NCplasma cutting apparatus. More particularly, the invention relates to aplasma cutting method and an NC plasma cutting apparatus best suited tocutting a workpiece by generating a transferred plasma arc.

BACKGROUND ART

Hitherto, a plasma cutting method has been widely employed as aprocessing method for cutting plate workpieces, such as iron, stainlesssteel, aluminum and the like, at high speed. Plasma cutting apparatusesused for this method are classified into a transferred type and anon-transferred type, depending on the method of generating ahigh-temperature plasma arc. A typical non-transferred plasma cuttingapparatus, as schematically shown in FIG. 6, comprises an electrode 11disposed at the center of a plasma torch and a funnel-shaped nozzle 12which is placed to surround the forward end of the electrode 11 across aplasma gas passage and which has an opening at the forward end thereof.Both the electrode 11 and the nozzle 12 are connected to the outputterminal of a power supply source 16. With this construction, during acutting operation, an arc discharge is generated between the electrode11 and the nozzle 12 while a plasma gas is allowed to flow therebetween.The plasma gas is thus transformed into the state of high-temperatureplasma, and at the same time, a non-transferred plasma jet 13 is ejectedthrough the opening formed at the forward end of the nozzle 12 toward aworkpiece 14 to be cut, thereby melting and cutting the workpiece 14.This non-transferred plasma cutting apparatus is advantageous in thatthe workpiece 14 can be cut by this apparatus whether it is formed of anelectrically-conductive material or a non-conductive material. However,the apparatus presents a problem in that it incurs a greater loss of thenozzle and has a heat efficiency as low as approximately 20%.

On the other hand, a typical transferred plasma cutting apparatus, asschematically shown in FIG. 7, is basically constructed in a mannersimilar to the non-transferred plasma cutting apparatus. That is, thetransferred plasma cutting apparatus comprises an electrode 11 and anozzle 12, with each of the electrode 11, the nozzle 12, and theworkpiece 14 to be cut being connected to an output terminal of thepower supply source 16.

A description will now be given of a cutting operation of theapparatuses constructed as described above. At the start of theoperation, as illustrated in FIG. 6, arc discharge is effected betweenthe electrode 11 and the nozzle 12 so that a non-transferred plasma jet13 is ejected from the opening of the nozzle, thus generating a pilotarc. When this plasma jet 13 reaches the workpiece 14, electricalconductivity can be obtained between the electrode 11 within the torchand the workpiece 14. Subsequently, as illustrated in FIG. 7, a currentflows between the electrode 11 and the workpiece 14, and simultaneously,the nozzle 12 and the power supply source 16 are disconnected so thatthe pilot arc can be extinguished, and a transferred plasma jet 15 canbe formed. The starting operation of the transferred plasma arc is thuscompleted.

Upon completion of the starting operation, the transferred plasma jet 15starts to cut the workpiece 14. For achieving high performance ofcutting, the plasma arc current applied during the cutting operation isset as the optimum value in consideration of various conditions, such asthe shape of the plasma torch, the type of plasma gas, the type and thethickness of the workpiece to be cut, cutting speed and otherconditions. The power supply source 16 is controlled so that thisoptimum current value can be maintained. In general, this plasma arccurrent is set to be constant without being changed until the cuttingoperation is completed. In contrast, when a cutting operation isperformed on a workpiece having the shape of a small circular arc or anacute angle in cross-section, it is required that the speed of a torchmoving device be decreased for preventing vibration of the torch movingdevice and ensuring locus precision. For achieving these requirements,it is known that the following measures are taken. The plasma arccurrent is reduced in accordance with a decrease in the moving speed ofthe torch, thus preventing excessive heat from being transferred to theworkpiece, thereby obtaining good cutting quality, in particular, thecutting quality exhibiting good appearance of corner portions (forexample, refer to Japanese Utility Model Laid-Open No. 1-165171).

Then, upon completion of the cutting operation, the transferred plasmaarc current is interrupted, generally at a cutting finishing point.After the current interruption, the generation of heat required forperforming the cutting operation is terminated, and the workpiece stopsmelting, thus completing the cutting operation.

On the other hand, the following technique is known in contrast to theabove-described cutting method. Upon completion of the cuttingoperation, the non-transferred pilot arc is ignited before thetransferred plasma arc is extinguished so that the above-described pilotarc can be maintained at the cutting finishing point even after thetransferred plasma arc has been extinguished, thus enabling speedystarting of a subsequent cutting operation (for example, refer toJapanese Patent Laid-Open No. 63-5875 and Japanese Patent PublicationNo. 4-9637).

The typical transferred plasma cutting operation has thus been explainedas described above. A higher heat efficiency of approximately 80% can beachieved by use of this method. Accordingly, this method is oftenemployed in the metal processing field in which a workpiece is formed ofa good conducting material.

When a cutting operation is performed by the transferred plasma arc, forexample, when a bore-cutting operation is performed on a metal workpieceto make a hole therein in the shape of a circle, ellipse, rectangle orthe like, the cutting operation is generally performed according to alocus shown in FIG. 8. Referring to FIG. 8, reference numeral 24 denotesa cutting line; 21 indicates a cutting starting point; and 22 designatesa cutting finishing point. The cutting operation is started bygenerating a transferred plasma arc at the cutting starting point 21,and then, the cutting operation is performed along the cutting line 24,followed by interrupting the transferred plasma arc current at thecutting finishing point 22, thus accomplishing the cutting operation.The reason that the cutting finishing point 22 is not set at a crossingpoint 23 on the cutting line 24 will be given below. If the cuttingfinishing point 22 is set at the crossing point 23, it is necessary toprecisely provide timing for the movement of the plasma torch and theinterruption of the arc current. If the arc current is interrupted tooearly, the workpiece is left partially uncut. In order to avoid thisdisadvantage, the cutting line 24 is extended inwardly toward the holeso that a reliable cutting operation can be ensured.

However, in the above-described bore-cutting operation, when the size ofa hole to be cut is small, a workpiece scrap 25 is frequently droppedwithout being held by a workpiece-supporting portion of the cuttingapparatus. This causes a discharge point of the transferred plasma arcto remain on the inner surface of the hole formed in a product 26, asillustrated in FIG. 9. This arc discharge continues until the dischargearc is naturally extinguished due to the subsequent movement of theplasma torch. More specifically, the plasma torch is moved away from theinner surface of the hole so as to cause an increase in the arc voltage,which makes it impossible to maintain discharging. Accordingly, untilthe arc discharge is extinguished, the inner surface of the hole isheated by the arc discharge, and also, it is likely that a double arcwill be generated. This increases the degree of melting, burning or thelike, occurring on the inner surface of the hole formed in the product26, which further causes a problem of a deterioration in the cuttingquality.

The following measure is known to solve the above-described problem.Examination has been made on the arc voltage which is changed during thecutting operation depending on various conditions, such as the arccurrent value, the gas pressure, the cutting speed, and otherconditions. A set value of the arc voltage has then been determined.Thus, taking advantage of an increase in the arc voltage after theabove-described workpiece scrap has been dropped, the plasma arc currentis interrupted when the voltage exceeds the set value, thus shorteningthe duration of discharging applied to the workpiece. With thisarrangement, a deterioration in the cutting quality can thus beeffectively inhibited (for example, Japanese Patent Laid-Open No.1-241379).

However, in this method of taking advantage of the cutting arc voltage,it is necessary to predict a change in the voltage, such as an increasein the arc voltage depending on the cutting shape of, for example, asmall circular arc, an acute angle, or the like. Additionally, forpreventing a stoppage of the cutting operation with respect to the arcvoltage, it is necessary that a somewhat higher set value be determined.Because of these requirements, it will take a longer time to interruptthe plasma arc current, whereby it is likely that the burning will beleft on the cutting surface.

SUMMARY OF INVENTION

Accordingly, in order to overcome the above-described drawbacks inherentin the prior art, an object of the present invention is to provide aplasma cutting method and an NC plasma cutting apparatus in which animprovement can be made in the quality of the working surface in thevicinity of the cutting finishing point when a cutting operation isperformed by use of a transferred plasma arc.

According to a first aspect of the present invention, there is provideda plasma cutting method for switching from a transferred plasma arc to anon-transferred plasma arc at a position immediately before a cuttingfinishing point and in the vicinity of a position at which cutting linescross each other. Also, simultaneously with the switching operation tothe non-transferred plasma arc, a moving speed of a plasma torch can bevaried in accordance with a change from a transferred plasma arc currentto a non-transferred plasma arc current.

With this construction, even when a workpiece scrap is separated anddropped from a cut hole formed in a workpiece, a discharge point hardlyremains in the resultant product. This can prevent a larger amount ofheat than is necessary from being transferred to the product, thusobtaining a cutting surface of good quality. Further, the moving speedof the plasma torch can be varied, for example, the moving speed can bedecreased to compensate for the lower heat efficiency of thenon-transferred plasma arc, thus attaining suitable cutting capability.

According to a second aspect of the present invention, there is provideda plasma cutting method for reducing a transferred plasma arc current ata position immediately before a cutting finishing point and in thevicinity of a position at which cutting lines cross each other. Also,simultaneously with reducing the transferred plasma arc current, amoving speed of a plasma torch can be varied in accordance with thereduction in the current.

With this construction, even though a discharge point remains in theresultant product when a workpiece scrap is separated and dropped from acut hole, the discharge point exhibits only a small plasma arc current,thus decreasing the heating value. Consequently, the product can beinhibited from melting, which further minimizes a deterioration in thequality of the cutting surface. In accordance with the aforementionedadvantage, a decrease in the moving speed or the like can be realized tocompensate for a reduction in the heating value due to a decrease in theplasma arc current, thereby attaining suitable cutting capability.

According to a third aspect of the present invention, there is provideda plasma cutting method in which a function of detecting the voltage iskept in the off state during a cutting operation and is actuated at aposition immediately before a cutting finishing point and in thevicinity of a position at which cutting lines cross each other. Also, afunction of interrupting the plasma arc current can instead be kept inthe off state during the cutting operation and actuated at theabove-described position.

With this arrangement, it becomes possible to set a suitable voltagerequired for interrupting the arc current, without having the necessityof considering a change in the arc voltage caused by various factorsduring the cutting operation until reaching a position immediatelybefore the cutting finishing point. This eliminates the necessity ofsetting an actual voltage higher than a suitable voltage, whichconventionally has been necessary. Hence, it becomes possible to shortenthe time required from dropping a workpiece scrap until the arc currentis interrupted, thereby decreasing the heating value at a dischargepoint remaining in the resultant product, resulting in good quality ofthe cutting surface. With this method, it is possible to achieve theobject more accurately, in comparison with a method of interrupting thearc current by detecting an increase in the arc voltage (for example,the above-discussed Japanese Patent Laid-Open No. 1-241379). Also, theplasma arc current interrupting function can be applied, in which case,operations and advantages similar to those obtained by the voltagedetecting function can be attained.

According to a fourth aspect of the present invention, there is providedan NC plasma cutting apparatus comprising means for inputting, inadvance into a control program, data concerning a position immediatelybefore a cutting finishing point and at which cutting lines cross eachother; means for detecting, during a cutting operation, that theposition which has been inputted into the control program has nearlybeen reached; and means for giving an instruction, based on the resultof the detection, for performing the cutting operation which should beexecuted. The instruction means can be means for giving an instruction,based on the result of the detection, for switching from a transferredplasma arc to a non-transferred plasma arc. Alternatively, theinstruction means can be means for giving the switching instruction andalso for giving an instruction for varying a moving speed of a plasmatorch in accordance with a change from a transferred plasma arc currentto a non-transferred plasma arc current. Further, the instruction meanscan be means for giving an instruction, based on the result of thedetection, for reducing a transferred plasma arc current. Alternatively,the instruction means can be means for giving the current-reducinginstruction and also giving an instruction for varying a moving speed ofa plasma torch in accordance with the current reduction. Further, theinstruction means can be means for giving an instruction, based on theresult of the detection, for allowing a voltage detecting function to bekept in the off state during the cutting operation and for actuating thefunction at the position immediately before the cutting finishing pointand in the vicinity of the position at which the cutting lines crosseach other. Alternatively, the voltage detecting function can besubstituted by a plasma arc current interrupting function.

With this arrangement, it is possible to obtain an NC plasma cuttingapparatus which makes changes to the proceeding cutting operation at aposition immediately before the cutting finishing point and in thevicinity of a position at which cutting lines cross each other, such aposition being instructed by the program. By use of this apparatus, aseries of procedures of cutting processing can be automaticallyperformed, and at the same time, the quality of cut products can also beimproved. Further, in combination with the instruction means, thecutting processing can be performed to satisfy a demanded cuttingquality without requiring complicated and troublesome operations.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B show a timing chart of a transferred plasma arc and anon-transferred plasma arc according to a first aspect of the presentinvention;

FIG. 2 is a timing chart of a transferred plasma arc current accordingto a second aspect of the present invention;

FIGS. 3A and 3B show partial timing charts of a transferred plasma arccurrent and the moving speed of a plasma torch of another embodimentaccording to the second aspect of the present invention;

FIGS. 4, 4A an 4B show timing charts of a transferred plasma arc currentand an arc voltage detection device according to a third aspect of thepresent invention;

FIG. 5 is a flow chart in which control is executed over an NC plasmacutting apparatus according to a fourth aspect of the present invention;

FIG. 6 is a schematic view illustrative of the operation of a typicalnon-transferred plasma arc cutting apparatus according to a conventionalart;

FIG. 7 is a schematic view illustrative of the operation of a typicaltransferred plasma arc cutting apparatus according to a conventionalart;

FIG. 8 is a schematic view showing an example of a locus of a typicalbore-cutting operation; and

FIG. 9 shows that a bore-cutting operation is completed by use of atransferred plasma arc cutting apparatus according to a conventionalart.

BEST MODE FOR CARRYING OUT THE INVENTION

A detailed description will now be given of a preferred embodiment of aplasma cutting method according to a first aspect of the presentinvention with reference to the accompanying drawings.

FIG. 1 is a timing chart of this embodiment and shows a cutting startingpoint 1, an ignition point 2 of a non-transferred plasma arc, anextinguishing point 3 of a transferred plasma arc, a bore-cuttingfinishing point 4 and a cutting finishing point 5. This embodimentemploys an NC plasma cutting apparatus of the type which performs acutting operation based on a control program having normal cuttinginformation and cutting conditions shown in FIG. 1 inputted therein. Byuse of this apparatus, a cutting operation is performed according to alocus of a typical bore-cutting operation on a metal workpiece. Thisembodiment will be described with reference to FIGS. 1 and 8. In amanner similar to a typical cutting operation, the non-transferredplasma arc is ignited and extinguished so as to generate the transferredplasma arc at the position indicated by reference numeral 1corresponding to the cutting starting point 21, thereby starting thebore-cutting operation. The cutting operation is performed along acutting line 24 by the transferred plasma arc. Reference numerals 2 and3 are set so that the cutting-proceeding position can be located in thevicinity of the crossing point 23 on the cutting line 24. Morespecifically, the cutting-proceeding position is more preferably locatedat a position immediately before the crossing point 23 when highprecision is demanded for a cutting operation. On the other hand, thecutting-proceeding position is more preferably located at a positionimmediately after passing through the crossing point 23 when it isdesired that the cutting speed take priority over precision. When thereference numerals 2 and 3 are set at a position immediately before thecrossing point 23, the plasma torch, which has run generally one roundalong the cutting line 24, ignites the non-transferred plasma arc and atthe same time, extinguishes the transferred plasma arc at a positionimmediately before the crossing point 23, thereby switching from thetransferred plasma arc to the non-transferred plasma arc. After thisswitching operation, the plasma torch reaches the crossing point 23,which corresponds to reference numeral 4, on the cutting line 24 in ashort time, thus accomplishing the bore-cutting operation. Accordingly,the cutting operation is performed by the non-transferred plasma arcduring the interval between the positions indicated by referencenumerals 3 and 4, i.e., the locus from immediately before the crossingpoint 23 to the crossing point 23. Subsequently, at a position indicatedby reference numeral 5 corresponding to the cutting finishing point 22,an interruption signal is transmitted to a plasma supply source so thatthe non-transferred plasma arc is extinguished, thus accomplishing thecutting operation. It is not essential to provide a locus from thecrossing point 23 at which the bore-cutting operation is actuallyfinished to the cutting finishing point 22. Such an extra locus is,however, effective for ensuring a proper bore-cutting operation. In thenon-transferred plasma arc, discharging occurs between the electrode andthe nozzle of the plasma torch. Accordingly, it is operable even whenthere is no workpiece to be cut under the plasma torch, that is, whenthe workpiece scrap 25 is separated and dropped from the product 26. Inthe manner described above, the product 26 obtained by this embodimentdoes not leave any trace of a discharging point in the cut hole formedin the product 26 even when the workpiece scrap is dropped from the cuthole. It is thus possible to obtain a cutting surface of high quality.

A detailed explanation will now be given of a preferred embodiment ofthe plasma cutting method according to a second aspect of the presentinvention with reference to the accompanying drawings.

FIG. 2 is a timing chart of this embodiment. This embodiment employs anNC plasma cutting apparatus having cutting conditions shown in FIG. 2inputted into a control program. By use of this apparatus, abore-cutting operation is performed on a metal workpiece along a locusshown in FIG. 8 by virtue of a transferred plasma arc. This embodimentwill be explained with reference to FIGS. 2 and 8. First of all, thetransferred plasma arc is generated at a position indicated by referencenumeral 1 by use of the non-transferred plasma arc (not shown), therebystarting the bore-cutting operation from the cutting starting point 21.The operation is performed along the cutting line 24. Reference numerals6 and 7 are set so that the cutting-proceeding position can be locatedin the vicinity of the crossing point 23, and more preferably, at aposition immediately before the crossing point 23. During the intervalbetween reference numerals 6 and 7, the transferred plasma arc currentat a point indicated by 6 is reduced to such a degree as to reach alevel of the current indicated by reference numeral 8. In the state ofthe plasma arc with the reduced current, in a short time the plasmatorch reaches the crossing point 23, which is the bore-cutting finishingpoint, corresponding to reference numeral 4, thus accomplishing thebore-cutting operation. The reason that the transferred plasma arc isextinguished from the bore-cutting finishing point indicated by 4 onwardwill be given below. In the transferred plasma arc in which arcdischarging occurs between the electrode within the plasma torch and aworkpiece to be cut, there is an increase in the voltage when aworkpiece scrap is dropped, whereby the transferred plasma arc is unableto be maintained. It is needless to say, however, that the input timingchart can be modified in such a manner that the transferred plasma arcis not extinguished at the position indicated by 4, and instead, thecutting operation is continued at a level of the current indicated by 8,and the transferred plasma arc is extinguished at the position indicatedby 5 corresponding to the cutting finishing point 22, thus accomplishingthe cutting operation. In such a case, the transferred plasma arc isoften naturally extinguished before reaching the cutting finishing point22 due to an increase in the arc voltage.

In the manner described above, the current of the transferred plasma arcis reduced immediately before the crossing position 23 on the cuttingline 24 so that only a small amount of current flows at a dischargepoint which remains in the product even when the workpiece scrap isseparated and dropped from a hole formed by the cutting operation.Hence, only a very small trace of discharging is left, and thus, it ispossible to maintain the high quality of the cutting surface of theproduct.

In contrast to the cutting operation performed by reducing thetransferred plasma arc current in the above-described embodiment, adescription will further be given of an embodiment in which the movingspeed of the plasma torch is varied in accordance with the reduction inthe transferred plasma arc current. FIG. 3 shows one example of a timingchart of the time interval between reference numerals 6 and 7 shown inFIG. 2 illustrating the previous embodiment. In this embodiment, thetransferred plasma arc current is reduced, and simultaneously, themoving speed of the plasma torch is decreased in accordance with thischange in the current. With this arrangement, even when the cuttingcapability is changed due to a change in the plasma arc current,suitable cutting conditions can be obtained by varying the moving speedof the torch, and more preferably, by decreasing the moving speed, thusobtaining better quality of the cutting surface.

In the embodiment according to the first aspect of the presentinvention, as well as this embodiment, advantages similar to thoseobtained in this embodiment can be realized by the following process,which is more preferable. That is, the transferred plasma arc isswitched to the non-transferred plasma arc, and simultaneously, themoving speed of the plasma torch is changed, and more specifically, themoving speed is decreased, in accordance with the switching from thetransferred plasma arc current to the non-transferred plasma arccurrent.

A detailed description will now be given of a preferred embodiment of aplasma cutting method according to a third aspect of the presentinvention with reference to the accompanying drawings.

FIG. 4 is a timing chart of this embodiment. This embodiment employs anNC plasma cutting apparatus provided with an arc voltage detectiondevice for detecting a voltage of a transferred plasma arc andinterrupting a plasma arc current when the detected voltage exceeds aset value (for example, a device disclosed in the above-describedJapanese Patent Laid-Open No. 1-241379). By use of this apparatus, abore-cutting operation is performed on a metal workpiece along the locusshown in FIG. 8. This embodiment will be explained with reference toFIGS. 4 and 8. First of all, a transferred plasma arc is generated at aposition indicated by reference numeral 1 by a non-transferred plasmaarc, thereby starting a bore-cutting operation from the cutting startingpoint 21. The cutting operation is performed along the cutting line 24.From the start of the cutting operation, the voltage detecting functionof the arc voltage detection device has been turned off, that is, isinoperable. Subsequently, the voltage detecting function of the arcvoltage detection device is actuated at a position indicated byreference numeral 9, that is, at a position immediately before thecrossing point 23 of the cutting lines. With this arrangement, itbecomes possible to set a suitable voltage required for interrupting thearc current to complete a cutting operation, regardless of a change inthe arc voltage caused by various factors during the cutting operation,which further eliminates the necessity of setting an actual voltagehigher than a suitable voltage, which conventionally has been necessary.This can shorten the time required from the dropping of the workpiecescrap 25 until the arc current is interrupted, thereby minimizing anytrace of a discharge point left in a resultant product. It should benoted that the timing chart can be modified in a manner described below.The transferred plasma arc might not be extinguished at a positionindicated by reference numeral 4, which is the bore-cutting finishingpoint corresponding to the crossing point 23 on the cutting line 24, andinstead, the cutting operation might be continued while the transferredplasma arc is kept in the ignition state. Then, the plasma arc can beextinguished at a position indicated by reference numeral 5corresponding to the cutting finishing point 22, thus accomplishing thecutting operation. Additionally, in place of the voltage detectingfunction provided with the arc voltage detection device, a plasma arccurrent interrupting function provided with the arc voltage detectiondevice can be employed. In such a case, operations and advantagessimilar to those obtained by the foregoing embodiments can be attained.

A detailed explanation will now be given of a preferred embodiment of anNC plasma cutting apparatus according to a fourth aspect of the presentinvention with reference to the accompanying drawings.

FIG. 5 is a schematic flow chart illustrative of one embodiment in whichcontrol is executed over an NC plasma cutting apparatus, and morespecifically, a flow chart having a program in which a transferredplasma arc is switched to a non-transferred plasma arc at a positionimmediately before a cutting finishing point and in the vicinity of acrossing point of cutting lines. This apparatus is similar to typical NCplasma cutting apparatuses in that a cutting operation is performedalong a cutting shape from a cutting starting point based on theinputted program.

In step 51, data concerning a cutting shape, the cutting conditions, anda crossing point at a position immediately before the cutting finishingpoint are first read. The cutting conditions are set in step 52, and acutting operation is started in step 53. Subsequently, in step 54, it isdetermined whether a position at which the cutting operation iscurrently proceeding matches the data on the position in the vicinity ofthe crossing point of the cutting lines and the position immediatelybefore the cutting finishing point. The data on the crossing point onthe cutting lines can be represented by coordinates, by way of example.Also, a determination of whether a position at which the cuttingoperation is currently proceeding is near the above-describedcoordinates can be made by means such as determining whether such aposition is located in a region of being with a predetermined distanceaway from the coordinates. When it is determined that the position atwhich the cutting operation is currently proceeding is located in thevicinity of the crossing point 23, the flow proceeds to step 55 in whichthe transferred plasma arc is switched to the non-transferred plasmaarc. The cutting operation is started again based on the cutting shape(step 56). When the cutting finishing point 22 is reached, the powersupply source is turned off in step 57, thus accomplishing the cuttingoperation. On the other hand, when it is determined in step 54 that thecurrently-cutting position is not located in the vicinity of thecrossing point of the cutting lines, the flow proceeds to step 58 inwhich the cutting operation is continued based on the cutting shape, andthen, the flow returns to step 54. According to the process discussedabove, even when a workpiece scrap is separated and dropped from the cuthole, a trace of a discharge point is not left on the cut hole formed inthe resultant product, thereby obtaining a cutting surface of goodquality.

Although the switching operation from the transferred plasma arc to thenon-transferred plasma arc is performed in step 55 in this embodiment,the foregoing plasma cutting method according to the present inventioncan instead be inputted in step 55. For example, the below-describedinstructions are inputted. Simultaneously with the switching operationfrom the transferred plasma arc to the non-transferred plasma arc, themoving speed of the plasma torch is varied in accordance with a changefrom the transferred plasma arc current to the non-transferred plasmaarc current. The transferred plasma arc current is reduced.Simultaneously with this instruction for reducing the current, themoving speed of the plasma torch in accordance with the reduction in thecurrent is varied. The voltage detecting function is kept in the offstate during the cutting operation from the start of the cuttingoperation (step 51), and at a position immediately before the cuttingfinishing point 22 and in the vicinity of the crossing point 23 of thecutting lines, the above-mentioned function is actuated. The plasma arccurrent interrupting function is kept in the off state during thecutting operation from the start of the cutting operation (step 51), andat a position immediately before the cutting finishing point 22 and inthe vicinity of the crossing point 23 of the cutting lines, the abovementioned function is actuated. The above-described instructions can beinputted so as to obtain a cutting surface of good quality. Theinstructions can be combined with each other and inputted as required.With the NC plasma cutting apparatus, by application of theabove-discussed program, automatic cutting processing to satisfy demandsof various types of cutting qualities and cutting speeds can beachieved, thus eliminating the need for complicated and troublesomeoperations and improving productive efficiency.

INDUSTRIAL APPLICABILITY

The present invention is useful as a plasma cutting apparatus and aplasma cutting method in which an improvement can be made in the qualityof a bore-cut product by use of a transferred plasma arc, and also, theproductive efficiency can be enhanced.

We claim:
 1. A method for utilizing a plasma arc torch to cut a hole ina workpiece by cutting the workpiece along a cutting line which extendsfrom a cutting starting point to a cutting finishing point such thatsaid cutting line crosses itself at a crossover point, thereby definingsaid hole, said plasma arc torch comprising an electrode and a nozzle,said method comprising the steps of:generating a transferred plasma arccurrent between said electrode and said cutting starting point on theworkpiece, using said transferred plasma arc current to effect cuttingof said workpiece along said cutting line beginning from said cuttingstarting point, terminating said transferred plasma arc current andgenerating a non-transferred plasma arc current between said electrodeand said nozzle when the cutting of the workpiece reaches a position onsaid cutting line which is before said cutting finishing point and inthe vicinity of said crossover point, and using said non-transferredplasma arc current to cut said workpiece along said cutting line fromsaid switching point to said cutting finishing point.
 2. A method inaccordance with claim 1, wherein there is a speed of relative movementbetween said plasma arc torch and said workpiece during the cutting ofsaid workpiece, further comprising varying said speed in accordance witha change from said transferred plasma arc current to saidnon-transferred plasma arc current at said position.
 3. A method inaccordance with claim 1, wherein there is a speed of relative movementbetween said plasma arc torch and said workpiece during the cutting ofsaid workpiece, further comprising reducing said speed upon a changefrom said transferred plasma arc current to said non-transferred plasmaarc current at said position.
 4. A method for utilizing a plasma arctorch to cut a hole in a workpiece by cutting the workpiece along acutting line which extends from a cutting starting point to a cuttingfinishing point such that said cutting line crosses itself at acrossover point, thereby defining said hole, said plasma arc torchcomprising an electrode and a nozzle, said method comprising the stepsof:generating a transferred plasma arc current between said electrodeand said cutting starting point on the workpiece, using said transferredplasma arc current to effect cutting of said workpiece along saidcutting line beginning from said cutting starting point, reducing saidtransferred plasma arc current when the cutting of the workpiece reachesa position on said cutting line which is before said cutting finishingpoint and in the vicinity of said crossover point, and using the thusreduced transferred plasma arc current to cut said workpiece along saidcutting line from said position toward said cutting finishing point. 5.A method in accordance with claim 4, wherein the step of using the thusreduced transferred plasma arc current comprises using the thus reducedtransferred plasma arc current to cut said workpiece along said cuttingline from said position to said cutting finishing point.
 6. A method inaccordance with claim 4, wherein there is a speed of relative movementbetween said plasma arc torch and said workpiece during the cutting ofsaid workpiece, further comprising varying said speed in accordance withthe reduction of said transferred plasma arc current at said position.7. A method in accordance with claim 4, wherein there is a speed ofrelative movement between said plasma arc torch and said workpieceduring the cutting of said workpiece, further comprising reducing saidspeed upon the reduction of said transferred plasma arc current at saidposition.
 8. A method for utilizing a plasma arc torch to cut a hole ina workpiece by cutting the workpiece along a cutting line which extendsfrom a cutting starting point to a cutting finishing point such thatsaid cutting line crosses itself at a crossover point, thereby definingsaid hole, said plasma arc torch comprising an electrode and a nozzle,said method comprising the steps of:generating a transferred plasma arccurrent between said electrode and said cutting starting point on theworkpiece, using said transferred plasma arc current to effect cuttingof said workpiece along said cutting line beginning from said cuttingstarting point, when the cutting of the workpiece reaches a position onsaid cutting line which is before said cutting finishing point and inthe vicinity of said crossover point, activating a function of detectinga voltage between said workpiece and one of said electrode and saidnozzle as said workpiece is being cut, and interrupting said transferredplasma arc current when the thus detected voltage exceeds a set value.9. A method in accordance with claim 8, wherein said function ofdetecting said voltage is deactivated during the cutting from saidstarting point to said position.
 10. A method for utilizing a plasma arctorch to cut a hole in a workpiece by cutting the workpiece along acutting line which extends from a cutting starting point to a cuttingfinishing point such that said cutting line crosses itself at acrossover point, thereby defining said hole, said plasma arc torchcomprising an electrode and a nozzle, said method comprising the stepsof:generating a transferred plasma arc current between said electrodeand said cutting starting point on the workpiece, using said transferredplasma arc current to effect cutting of said workpiece along saidcutting line beginning from said cutting starting point, detecting avoltage between said workpiece and one of said electrode and said nozzleas said workpiece is being cut, and when the cutting of the workpiecereaches a position on said cutting line which is before said cuttingfinishing point and in the vicinity of said crossover point, activatinga function of interrupting said transferred plasma arc current when thethus detected voltage exceeds a set value.
 11. A method for controllinga cutting operation utilizing a plasma arc torch to cut a hole in aworkpiece by cutting the workpiece along a cutting line which extendsfrom a cutting starting point to a cutting finishing point such thatsaid cutting line crosses itself at a crossover point, thereby definingsaid hole, said plasma arc torch comprising an electrode and a nozzle,said method comprising the steps of:providing a control program havingcutting information, inputting, in advance into said control program,data concerning a position on said cutting line which is before saidcutting finishing point and in the vicinity of said crossover point,generating a plasma arc current between said electrode and said cuttingstarting point on the workpiece, using said plasma arc current to effectcutting of said workpiece along said cutting line beginning from saidcutting starting point, detecting, during the cutting operation, thatsaid position has nearly been reached by the cutting; and utilizing saidcontrol program to give an instruction, based on the result of saiddetecting, for modifying the cutting operation by reducing said plasmaarc current.
 12. A method in accordance with claim 11, wherein there isa speed of relative movement between said plasma arc torch and saidworkpiece during the cutting of said workpiece; and wherein the step ofutilizing said control program comprises utilizing said control programto give an instruction, based on the result of said detecting, forreducing said plasma arc current and for giving an instruction forvarying said speed in accordance with the reducing of said plasma arccurrent.
 13. A method for controlling a cutting operation utilizing aplasma arc torch to cut a hole in a workpiece by cutting the workpiecealong a cutting line which extends from a cutting starting point to acutting finishing point such that said cutting line crosses itself at acrossover point, thereby defining said hole, said plasma arc torchcomprising an electrode and a nozzle, said method comprising the stepsof:providing a control program having cutting information, inputting, inadvance into said control program, data concerning a position on saidcutting line which is before said cutting finishing point and in thevicinity of said crossover point, generating a plasma arc currentbetween said electrode and said cutting starting point on the workpiece,using said plasma arc current to effect cutting of said workpiece alongsaid cutting line beginning from said cutting starting point, detecting,during the cutting operation, that said position has nearly been reachedby the cutting; and utilizing said control program to give aninstruction, based on the result of said detecting, for modifying thecutting operation; wherein the step of utilizing said control programcomprises utilizing said control program to give an instruction, basedon the result of said detecting, for switching said plasma arc currentfrom a transferred plasma arc current to a non-transferred plasma arccurrent.
 14. A method in accordance with claim 13, wherein there is aspeed of relative movement between said plasma arc torch and saidworkpiece during the cutting of said workpiece; and wherein the step ofutilizing said control program further comprises giving an instructionfor varying said speed in accordance with the switching from saidtransferred plasma arc current to said non-transferred plasma arccurrent.
 15. A method for controlling a cutting operation utilizing aplasma arc torch to cut a hole in a workpiece by cutting the workpiecealong a cutting line which extends from a cutting starting point to acutting finishing point such that said cutting line crosses itself at acrossover point, thereby defining said hole, said plasma arc torchcomprising an electrode and a nozzle, and method comprising the stepsof:providing a control program having cutting information, inputting, inadvance into said control program, data concerning a position on saidcutting line which is before said cutting finishing point and in thevicinity of said crossover point, generating a plasma arc currentbetween said electrode and said cutting starting point on the workpiece,using said plasma arc current to effect cutting of said workpiece alongsaid cutting line beginning from said cutting starting point, detecting,during the cutting operation, that said position ham nearly been reachedby the cutting; and utilizing said control program to give aninstruction, based on the result of said detecting, for modifying thecutting operation; wherein the step of utilizing said control programcomprises utilizing said control program to give an instruction, basedon the result of said detecting, for activating a voltage detectingfunction, which prior thereto during the cutting operation had been keptin an off state.
 16. A method in accordance with claim 15, wherein thevoltage detecting function comprises detecting a voltage between saidworkpiece and one of said electrode and said nozzle as said workpiece isbeing cut, and interrupting said plasma arc current when the thusdetected voltage exceeds a set value.
 17. An NC plasma cutting apparatusfor controlling a cutting operation utilizing a plasma arc torch to cuta hole in a workpiece by cutting the workpiece along a cutting linewhich extends from a cutting starting point to a cutting finishing pointsuch that said cutting line crosses itself at a crossover point, therebydefining said hole, said plasma arc torch having an electrode and anozzle, said apparatus comprising:a control program having cuttinginformation; means for inputting, in advance into said control program,data concerning a position on said cutting line before said cuttingfinishing point and in the vicinity of said crossover point; said plasmaarc torch for generating a plasma arc current to effect cutting of saidworkpiece along said cutting line; means for detecting, during thecutting operation, that said position has nearly been reached by thecutting; and means for giving an instruction, based on the result ofsaid detecting, for controlling the cutting operation; wherein saidmeans for giving an instruction gives an instruction, based on theresult of said detecting, for switching said plasma arc current from atransferred plasma arc current to a non-transferred plasma arc current.18. Apparatus in accordance with claim 17, further comprising means forproviding a moving speed of the plasma arc torch relative to saidworkpiece; and wherein said means for giving an instruction gives aninstruction, based on the result of said detecting, for switching saidplasma arc current from a transferred plasma arc current to anon-transferred plasma arc current, and also an instruction for varyingsaid moving speed in accordance with the switching from the transferredplasma arc current to the non-transferred plasma arc current.
 19. An NCplasma cutting apparatus for controlling a cutting operation utilizing aplasma arc torch to cut a hole in a workpiece by cutting the workpiecealong a cutting line which extends from a cutting starting point to acutting finishing point such that said cutting line crosses itself at acrossover point, thereby defining said hole, said plasma arc torchhaving an electrode and a nozzle, said apparatus comprising:a controlprogram having cutting information; means for inputting, in advance intosaid control program, data concerning a position on said cutting linebefore said cutting finishing point and in the vicinity of saidcrossover point; said plasma arc torch for generating a plasma arccurrent to effect cutting of said workpiece along said cutting line;means for detecting, during the cutting operation, that said positionhas nearly been reached by the cutting; and means for giving aninstruction, based on the result of said detecting, for controlling thecutting operation; wherein said means for giving an instruction gives aninstruction, based on the result of said detecting, for reducing theplasma arc current.
 20. Apparatus in accordance with claim 19, furthercomprising means for providing a moving speed of the plasma arc torchrelative to said workpiece; and wherein said means for giving aninstruction gives an instruction, based on the result of said detecting,for reducing the plasma arc current, and also an instruction for varyingsaid moving speed in accordance with the reducing of the plasma arccurrent.
 21. An NC plasma cutting apparatus for controlling a cuttingoperation utilizing a plasma arc torch to cut a hole in a workpiece bycutting the workpiece along a cutting line which extends from a cuttingstarting point to a cutting finishing point such that said cutting linecrosses itself at a crossover point, thereby defining said hole, saidplasma arc torch having an electrode and a nozzle, said apparatuscomprising:a control program having cutting information; means forinputting, in advance into said control program, data concerning aposition on said cutting line before said cutting finishing point and inthe vicinity of said crossover point; said plasma arc torch forgenerating a plasma arc current to effect cutting of said workpiecealong said cutting line; means for detecting, during the cuttingoperation, that said position has nearly been reached by the cutting;and means for giving an instruction, based on the result of saiddetecting, for controlling the cutting operation; wherein said means forgiving an instruction gives an instruction, based on the result of saiddetecting, for activating a voltage detecting function, which priorthereto during the cutting operation had been kept in an off state. 22.Apparatus in accordance with claim 21, wherein the voltage detectingfunction comprises detecting a voltage between said workpiece and one ofsaid electrode and said nozzle as said workpiece is being cut, and meansfor interrupting said plasma arc current when the thus detected voltageexceeds a set value.